In optical telecommunication networks that use wavelength division multiplexing (WDM), multiple optical channels are carried on a single optical fiber. The single optical fiber is included in an optical fiber link, which includes the optical fiber and optical amplifiers and any other optical components between two connecting points or nodes along an optical transmission line. Each channel operates at a different wavelength in the optical fiber. New channels may be added and existing channels dropped in a node using an optical add/drop multiplexing (OADM) module.
Optical amplifiers are used to amplify WDM signals which are transmitted in a long (up to a few thousands of kilometers) optical fiber link. These amplifiers are used both in-line and at the transmitting and receiving end of an optical fiber link. In particular, erbium-doped fiber amplifiers (EDFAs) have become a well-accepted key enabler of WDM optical communication networks. The gain dynamics of EDFAs are generally considered to be slow in comparison to other gain media, and are a result of the long spontaneous lifetime of excited erbium ions (about 10 ms). For transmission of high-speed data, the gain of an EDFA is advantageously undisturbed by the signal modulation. The slow gain dynamics, however, also result in a slow response time. In the presence of a large instantaneous change in input power to the EDFA, the slow response time can cause adverse effects downstream.
For example, in optical networks with an optical add/drop multiplexing (OADM) module associated with each optical fiber link, the total input power to the optical amplifiers in the optical fiber link downstream of the OADM may fluctuate considerably due to changes in the number of channels in the network. This change in total input power induces gain fluctuations in the downstream optical amplifiers. Therefore, some kind of gain stabilization method is commonly used to keep gain constant in the optical amplifier while the input optical power fluctuates. During the gain stabilization process, the channels amplified by the optical amplifier may experience a much different gain from that before the input power change. This process is referred to as a transient effect.
In an extreme case, a fiber cut or equipment failure upstream from the OADM may occur, causing a transient effect on the transmission of surviving channels in the downstream link, i.e., in those channels added at the OADM. Due to the transient effect, the surviving channels may also experience transmission errors and, if severe enough, these errors may cause unwanted protection switching in optical networks. Protection switching is an automatic recovery feature for fiber or node failure, which functions by wrapping data traffic onto an alternate fiber link or switching the data in a failed channel to another channel.
One common solution to achieve stabilization of gain in the optical amplifiers is to integrate an automatic gain control (AGC) loop in the amplifier circuitry. Typically, gain is monitored by detecting optical power in and optical power out of an optical amplifier using optical taps before and after a gain stage. The actual gain is then calculated as the ratio of power out to power in. Adjustments to the actual gain by adjusting amplifier parameters to maintain a preferred operating gain of the optical amplifier can then be made in a feedback loop.
As an example of an optical amplifier, an EDFA is an optical fiber with a fiber core material doped with Erbium rare earth ions. Optical gain in an EDFA is created when the optical fiber core is pumped by a laser diode, for example, to a level at which amplification occurs. The gain level of the EDFA depends on an inversion level of erbium ions excited by the laser diode, also referred to as the optical pump.
To compensate for a loss of signal caused by an upstream fiber cut or equipment failure, the AGC loop in each optical amplifier downstream of the OADM is typically used with an optical pump control method. In the pump control method, the inversion level in the EDFA is typically adjusted to maintain constant gain using control electronics. An operating current of the laser diode pump is typically adjusted, thus changing the optical pump power. Alternatively, the laser diode current is constant and the optical pump power is adjusted with the use of a variable optical attenuator in an optical feedback loop.
Though the pump control method may be adequate to correct slow or small fluctuations in amplifier gain over time, it is inadequate to handle transient effects on surviving channels resulting from fiber cuts or equipment failure upstream from the amplifier. The response time of the AGC loop depends on the response time of the gain dynamics of the EDFA, which is unacceptably slow. In other words, the slow dynamics of EDFA, though advantageous in that the gain is subsequently undisturbed by the modulation of the transmission signal, pose a problem in the event of a fiber cut or equipment failure.
The pump control method is particularly inadequate in the case of the large channel count drop that occurs when there is a fiber cut or equipment failure in an optically amplified WDM network. Two problems arise in this case. In a WDM network, each channel of communication operates at a particular wavelength. A spectral power distribution as well as the optical power at the input to the downstream optical amplifier changes, therefore, when fiber cuts or equipment failures occur. First, the sudden large drop in optical input power to an amplifier downstream of the OADM cannot be accommodated by the pump control method due to the slow gain dynamics of the EDFA. The surviving channels will, therefore, experience much different gain during the transient duration from that before the optical input power change. Second, the large change in the spectral power distribution results in a large change of stimulated Raman scattering (SRS) induced spectrum tilt in the downstream transmission fiber not compensated for by the pump control method. This is especially true for dense WDM transmissions.
SRS is a scattering phenomena, which occurs as a result of an incident wave scattering in a medium so that the incident wave is partially deflected to a higher wavelength. In an optical fiber supporting multiple wavelength-specific channels, energy from the channel with the lowest wavelength scatters into a next higher wavelength channel and so on. The result of this scattering phenomena is inter-channel signal mixing and a progressive increase in perceived gain in higher wavelength channels, which contributes to a spectrum tilt in the transmitted WDM signal. The spectrum tilt is further amplified as the WDM signal is transmitted through additional amplifiers and fiber in the network.
The SRS induced spectrum tilt changes as a result of both the abrupt change in the spectral power distribution of the input signal and the total optical input power change. SRS is an extremely fast (on the order of sub-picosecond) process. Consequently, both the transient gain and SRS spectrum tilt change could cause transmission errors for the channels transmitted to a downstream optical fiber link and undesired protection switching in the optical network.
Inserting a fast variable optical attenuator (VOA) into a multi-stage optical amplifier has been proposed to compensate the SRS induced spectrum tilt change. The control speed of VOA, however, cannot match the spectrum tilt change and the algorithm to simultaneously control both the pump power and VOA is very complicated and inadequately slow.
A second conventional method that has been proposed for transient control is the filler laser control method. The filler laser control method is to inject one or several high-power single frequency laser signals into the WDM transmission band at an OADM node. The output power of each filler laser is then adjusted to compensate the input power change due to fiber cut or network node failure upstream. Each filler laser occupies one WDM wavelength slot that could have been occupied by a transmission channel. Using multiple filler lasers occupying several WDM wavelength slots, therefore, reduces transmission capacity. Further, any filler lasers used must be inserted and spectrally filtered out at every OADM node.
In addition, a filler laser needs special modulation to avoid the stimulated Brillouin scattering that typically occurs when injecting high-power laser signals into a fiber. Brillouin scattering is caused by the interaction of light with acoustic waves in a media, resulting in some backscattered optical radiation. In a fiber, the backscattered radiation is guided and can result in efficient energy conversion of the input laser signal to the backscattered wave. The phenomenon ultimately limits the maximum optical power that can be launched into the fiber. Moreover, inhomogeneous broadening of optical amplifiers and the resulting spectral hole burning can cause gain variations at the optical wavelength of the filler laser, additionally limiting the capability of the filler laser control technique. As a result, the filler laser control method is complicated, costly, inefficient, and requires a reduction of the total transmission capacity of the optical network.
In summary, these and other conventional methods which have been employed in an attempt to mitigate the transient effect are complex, costly, and largely incapable of error free performance.
There is a need, therefore, for simple, low-cost, and high-performance transient control in dynamic optical networks. Specifically, a method and apparatus are needed for suppressing the transient effect and the related inter-channel SRS induced spectrum tilt change in the surviving optical channels, caused by upstream fiber cut or equipment failure.